In the interests of protecting the environment, the use of bleaching agents such as oxygen (O), one or other per-compound (P) such as hydrogen peroxide and ozone (Z) has recently been suggested for the bleaching of, for instance, sulphate pulp. This has led to the introduction, including on a commercial scale, i.e. full scale, of the use of this type of bleaching agent, including those referred to above, and also in the sequence stated above. By avoiding the use of bleaching agents containing chlorine, which in the final analysis give rise to corrosive chloride, it has proved increasingly possible to close the bleaching plants. The expression closing is used to denote that the washing fluids are handled to an increasing extent within the bleaching plant. In traditional open bleaching plants, the washing fluids (waste liquors) appearing after the respective bleaching stage, including after extraction (E) stage, are allowed to flow directly out to the recipient or, where appropriate, to an external purification plant.
It has emerged from the use of the oxidative bleaching agents exemplified above, and in particular from the use of some per-compound, that the content of metals in the pulp and/or even the presence of metals in general leads to problems. The metals that cause the most problems are the transition metals, of which manganese is the most problematical due to the presence of manganese in such large amounts. Manganese, for example, occurs naturally in the raw material, i.e. in the lignocellulose material, for example in the form of wood. The process water that is used also contains manganese as a general rule, and manganese can also originate from the apparatus used in the pulp manufacturing chain. In an attempt to deal with this problem, a complex forming stage (Q) has been introduced into the pulp treatment chain, preferably directly ahead of the peroxide bleaching stage. The addition of complexers such as EDTA, DTPA and NTA, and others at a suitable pH value, ensures that any free manganese ions are collected and, in particular, the manganese is converted from a fixed form in the pulp to a water soluble complexed form. Manganese complexes of the type Mn(EDTA).sup.2- or Mn(DTPA).sup.3- occur in this case. It is important, after this treatment stage, for the pulp to be washed extremely thoroughly so that no significant quantities of manganese complexes and any free complexers accompany the pulp into the peroxide bleaching stage. The waste liquor generated at this position, i.e. the washing fluid from the complex forming stage, has attracted particular attention of experts and is the subject of more detailed comment below.
A gradually increased closing of the bleaching plant and the pulp manufacturing process in its entirety with regard to the liquid circuit has, as previously indicated, both been proposed and implemented in practice. Different counter-current washing processes for the pulp have been proposed, and it has even been proposed that the washing fluid shall be conveyed in strict counter-current. What is meant by this is that clean washing fluid is introduced into the wash after the final bleaching stage, and that this is conveyed as a counter-current with a constantly increasing degree of contamination through the entire bleaching plant with all its washes and into the unbleached pulp for the purpose of washing that, too, before being conveyed finally, together with the spent digestion liquor making up weak liquor, to the evaporation plant prior to subsequent incineration in the recovery boiler.
In the case of a strictly counter-current liquid circuit through a bleaching plant containing a conventional complex forming stage, it has been assumed (and feared) that the manganese complexes in the solution (washing liquor) are broken down at one or more positions, with the result that the manganese becomes attached to the pulp once more and/or is not removed from the system. It has even been claimed that such a process cannot be avoided. This misgiving has resulted in that hitherto presented proposals for closing the liquid circuit in conjunction with the manufacture of, for example, bleached sulphate pulp include a separate treatment of the washing liquid from the complex forming stage. Examples of proposals include a separate evaporation and destruction of the washing liquid at great expense, and the use of the washing liquid to dissolve the smelt from the recovery boiler.